1. Field of the Invention
The present invention generally relates to digital video broadcasting (DVB). More specifically, the present invention relates to reducing Peak to Average Power Ratio (PAPR) in multi-carrier modulation based DVB systems.
2. Background Art
As known to those of skill in the art, DVB is the European consortium standard for the broadcast transmission of digital terrestrial television. DVB systems transmit a compressed digital audio/video stream, using multi-carrier modulation, such as orthogonal frequency division multiplexing (OFDM). Another popular method of transmitting signals is digital video broadcasting-terrestrial (DVB-T). When broadcasters employ this method, the transmitted signals do not travel via cable. Instead, they move from aerial antenna to aerial antenna, and from a signal blaster to home receiver.
DVB-T broadcasters transmit data using a compressed digital audio-video stream, with the entire process based on a Moving Picture Expert Group (MPEG)-2 standard. These transmissions can include all kinds of digital broadcasting, including high definition television (HDTV) and other high-intensity methods. This is an improvement over the old analog signals, which required separate streams of transmission.
Impulse noise is a growing concern in DVB-T transmissions because of its negative impact upon transmitted digital data and is common in OFDM based systems. Although OFDM based DVB-T system receivers provide a level of inherent immunity against impulse noise, this immunity is insufficient to ensure that digital TV signals are reliably transmitted in accordance with applicable standards. As an example, OFDM systems include data that is transmitted on multiple carriers simultaneously. In real life, when data is transmitted, it is passed through a data channel. This data channel subsequently manifests itself as a multiplicative impairment (e.g., impulse noise) on one of more of these carriers when they are received on a receiver side.
While impulse noise has traditionally been ignored in North American TV receiver market, several standards/specifications in the European market require high tolerance to impulse noise. Another troublesome aspect of OFDM systems is PAPR, which impacts the presence and effects of impulse noise. By reducing PAPR, the effects of impulse noise can correspondingly be reduced.
Some of the traditional techniques for reducing PAPR in OFDM systems include, by way of example, tone reservation, tone injection, or adaptive constellation extension. Each of these techniques is well understood by those of skill in the relevant art and will not be discussed in depth herein. All of these techniques, however, cause either a loss in data rate or an increase in the signal-to-noise ratio (SNR) required for reception. These techniques, therefore, have only limited utility.
To assist a receiver in overcoming multi-path distortion, pilot signals with known data patterns are transmitted. The pilot signals, sometimes called pilot tones or simply pilots, are used to support channel estimation operations. Traditional channel estimation operations attempt to estimate the amplitude and phase distortion introduced by the communications channel and can thus compensate for the SNR losses discussed above. Although more effective than many of the other techniques, channel estimation operations are not precise enough to completely compensate for all SNR losses.
What is needed, therefore, are improved methods and systems for reducing PAPR in OFDM based DVB-T system receivers that can compensate for corresponding losses in SNR.